A front collision of one vehicle colliding with another vehicle is generally detected by an acceleration sensor located in a front portion or a middle portion of the one vehicle. Acceleration of the one vehicle changes rapidly during the collision and the acceleration sensor hence detects the rapid change of the acceleration. When it is determined based on the detection of the acceleration sensor that the collision has occurred, a passenger protection device such as an air bag or a pretensioner device of a seat belt may be activated to protect a passenger from the collision.
A lateral collision of one vehicle sometimes occurs in which a front portion of another vehicle collides with a lateral side portion (for example, a door) of the one vehicle. When another vehicle collides with a relatively softer portion of the one vehicle than a pillar of the one vehicle, the change of the acceleration is not so significant. Therefore in this case, it is difficult to detect the collision by using the acceleration sensor.
A pressure sensor may be located in an interior space of a door of the vehicle. When the lateral impact collision has occurred, a pressure in the interior space of the door rapidly changes in a short period. The pressure sensor hence detects the rapid change of the pressure. An environmental pressure around the vehicle changes depending on height above sea level, and the interior space is not airtight in may cases. The pressure in the interior space therefore slowly changes in accordance with the environmental pressure (also referred to as a surrounding pressure). It is hence necessary to take into account change of the environmental pressures in order to detect the collision by using a pressure sensor detecting an absolute pressure.
JP 2004-508558A (corresponding to U.S. Pat. No. 6,885,966) discloses a pressure sensor for generating a signal which depends on the change of the environmental pressure, and a signal processing unit which is connected with a rear side of the pressure sensor. In the prior art, it is supposed that the pressure changes within a range from 97 percents of 600 hPa to 120 percents of 1200 hPa provided that the environmental pressure changes within the range from 600 hPa to 1200 hPa. Thus, a pressure within the range from 582 hPa to 1440 hPa is detected by the pressure sensor.
In the prior art, a threshold used for determining whether the collision has occurred varies depending on the environmental pressure. For example, the environmental pressure is set to th1 when the environmental pressure is a1, and the environmental pressure is set to th2 (≠th1) when the environmental pressure is a2 (≠a1). In the case that the environmental pressure is a1, it is determined that the collision has occurred when an output value of the pressure sensor exceeds the threshold th1. In the case that the environmental pressure is a2, it is determined that the collision has occurred when the output value exceeds the threshold th2. However in this prior art, the threshold must vary among many values and different comparisons between the output value and the threshold are made depending on the varying threshold.